The main objectives of Project 2 are to identify primary HIV-1 envelope protein (Env) that have high ability in eliciting antibodies against the CD4 binding site (CD4bs) and, if successful, to further select those Env antigens that can also elicit broad neutralizing activities through the induction of anti-CD4bs antibodies. Aim 1 To Identify variation in the antigenicity of the CD4bs for primary Envs of HIV-1. CD4bs is a key target for eliciting broad neutralizing antibodies (NAbs) against HIV-1, as shown by previous studies that had used mAb b12, and more recently, using sera from HIV infected individuals. Our recent studies have shown that not every HIV primary Env antigen is equally effective in inducing NAb responses. In Aim 1, we will test whether Envs from primary isolates of different backgrounds, phenotypes, and clades may have varying antigenicity for both neutralizing and non-neutralizing CD4bs antibodies. Aim 2 To understand the consequences of CD4bs antigenic variation on the neutralization sensitivity of primary HIV-1 isolates. It is well known that heavy glycosylation and epitope masking play a key role in resistance to neutralization. Beyond these generalizations however, specific determinants of neutralization sensitivity or resistance are difficult to identify due to the complexity of the Env and the often unknown interactions and changes in conformation that occur as a result of trimerization of Env. In Aim 2, we intend to determine if there is a link between CD4bs antigenicity and neutralization sensitivity. Aim 3 To determine the immunogenicity of primary envelopes with varying levels of CD4bs antigencity and to select Env antigens that can elicit anti-CD4bs mediated neutralizing activities. The precise characteristics of a good HIV immunogen have yet to be identified. Previous studies have made a number of attempts to increase the immunogenicity of the HIV Env with limited success. In Aim 3, we will test whether the CD4bs antigenicity may be one, as of yet unidentified, determinant of a good immunogen. Immunogenicity studies will be conducted with the DMA prime-protein boost approach to identify those primary Env antigens that can elicit strong anti-CD4bs antibodies, possibly with broad neutralizing activities as well. RELEVANCE (Seeinstructions): We plan to study the unique structure of HIV outside coat to find a common pattern on the coat that will allow scientists to use them as part of the vaccine components to induce good protective immune responses.